Step-back motor-control system



June 11, 1929.

Filed Nov. 1, 1926 Fig.1

Inventor:

Patented June 11, 1929.

UNITED STATES PATENT OFFICE.

BENJAMIN W. JONES, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELEC- TRIC COMPANY, A CORPORATION OF NEW YORK.

STEP-BACK MOTOR-CONTROL SYSTEM.

Application filed November 1, 1926.

Electric motors have heretofore been protected from temporary overloads by automatically re-inserting resistance in the motor circuit in response to the current taken by the motor so that a fly wheel or other mass in which energy has been stored may give up its energy and thus assist the motor to carry the load until the temporary condition has passed. Such systems are called step-back or notch-back systems. One of the ditliculties encountered in such systems is that when the resistance is thus re-inserted in the motor circuit, the value of motor current drops and unless provision is made therefor, the device responsive to the motor current effects the immediate shunting of the resistance. The fly-Wheel is thus unable to materially assist the motor in carrying the temporary overload. My invention is directed to the provision of simple, efi'ective and reliable means whereby after the resistance is thus re-inserted in the motor circuit, a substantial and definite time interval mustelapse before the resistor is again shunted. This gives the fly Wheel an opportunity to give up its stored energy and the resistance will not be automatically shunted until the motor current has been stabilized at a predetermined sait'e value.

More specifically, the presentinvcntion is directed to im n'ovements of the s stem of this general nature described and claimed in my Patent No. 1,614.198, dated January 11, 1927. In the system of said patent, electro-responsive means is provided for automatically preventing for a definite time interval the shunting of the resistor in the motor circuit after the resistor has been re-inserted therein in response to a temporary overload. The present invention is directed to the provision of a simpler, less expensive and equally effective arrangement whereby the said time interval is secured.

In carryin the invention into eifect in the form which I now regard as the preferred form thereof. I provide an electromagnetic time element device which consists of an electromagnct having a winding and a magnetic movable member under the control thereof. The said winding is energized when the resistor in the motor circuit is shunted, and when the resistor is re-inserted in the motor circuit, the said magnetic member moves to a position to prevent the shunting of the resistor for a definite time interval and then re- Serial No. 145,418.

leases to permit the shunting of the same. The definite time interval is obtained by the shunting of the winding of the electromagnetic device when the resistor has been re-iaserted in the motor circuit, and the inductive effect of the Winding prevents the flux set up thereby from decreasing immediately. This decreasing magnetic flux exerts a continuing holding effect on the said magnetic member that holds the same in the position to prevent the shunting of the resistor for a sufficient interval of time to permit the fly wheel to give up part of its stored energy.

For a better understanding of the invention, reference is had to the acompanying drawing, in which Fig. 1 shows a very simplified diagram of a system of control for an electric motor in acordance with the invention, and Fig. 2 is a perspective view of an overload relay provided with an electromagnetic time element device for preventing operation of the relay contacts to effect the shortcircuiting of the resistor in the motor circuit for a definite interval of time after the resistor has been reinserted therein.

In Fig. 1 the electric motor 10 to be controlled is indicated as of the direct currentseries type arranged to be energized from the direct current source of supply 11. The resistor 12 is included in the armature circuit of the motor for protecting the motor from ten'iporary overloads and the electromagnetic switch or contactor 13 is provided for shunting this resistor. The motor is connected to the source of supply by means of the line contactor 14, the winding of which is controlled by the master switch 15. The resistor shunting contactor 13 is controlled by means of the relay shown in Fig. 2 and this relay is indicated more or less diagrammatically in Fig. 1.

The relay comprises a switch member 16 which is biased to its closed position and is operated to its open position by means of the series coil 17. lVhen the switch member is thus operated to its open position, iteffects the deenergization of the resistor shunting contactor 13 so as to cause the main contacts of this contactor to open and reinsert the resistor 12 in the armature circuit of the motor. The reinsertion of this resistance effects a temporary reduction in the motor current and I have, therefore, provided a simple and effective electromagnetic time element device to prevent the reclosure of the switch member 16 for an interval of time after this switch member has been operated to its open position.

Th s time interval is provided by means of 'theelectromagnet 18 which has a movable n'iagnetic member or armature 19 servlng as a stop to arrest thereturn of the lever 20 asso' ciated with theswitch member 16 when the switch member 16 has been operated to its open position. tromagnet is controlled by an auxiliary switch 22 associatedwith the resistor contactor 13.

In general, the arrangement shown in Fig. 1 is such that when the line switch 1st is closed and the resistor conta'ctor 1 3 is closed the winding 21 will be energized so that when an overload occurs and the switch member .16 is operated to its open position, the armature 19will move clockwise to its attracted position, so as to thereby serve as a stop for the lever 20. The opening of the circuit through the switch member 16 will eli'cct the d-ecnergi- Za'tion of the resistor contactor 1?) so that this contactor will. automatically open and rein sert the resistor 12 in the a-rn'ia'tu're circuit of the motor. The auxiliary switch member associated with resistor contactor 13 will be returnedto its normal closed position by reason of the opening ofc'ontactor 13 and the winding 21 will thus be included in a local circuit. Although this winding is disconnected from the source of supply by the open spring 23, which biases the armature 19 to its unattracted position, overpower-s the magnetic holding eii'ect ot' the decreasing flux, the armature will return to its position indicated in Fig. 1, thereby permitting the switch member 16 of the overloaddevice to return to the closed positionthere'ot', in case the current taken by thermotor has dropped to the predetermined value. v v

It will be understood by those skilled in the art that the motor is connected to the source of supply by the closing of the master switch 15 which etlects the closing of the line contactor' 14. lVhen thisline contactor closes, the auxiliary switch 22 associated therewith also closes and the winding of resistor contactor 13 is energized through the contacts of the overload device. It will be understood that I have not shown anyacceleration controlling means for the motor, since systems for accelerating electric motors are in themselves quite old. The resistor 12 may be used to not only protect the motor when the current taken thereby is in excess or a certain value after the motor has been started,but

The winding 21 of this elc also may be used for protecting the motor durin the startin interval.

It will also be understood that while I It will be appreciated by those skilled in the art that I have provided a very simple system incorporating therein a very simple,

inexpensive and thoroughly reliable time element device which prevents the shunting 01' the protective resistor in the motor circuit for a definite interval of time after this resistance The magnetic blow-out device 25 of customary form is provided for interrupting the arcs tormed at the relay contacts. The switch member 16 is connected to be operated by the plungertnot shown) of an electromagnet which includes'the magnetic frame 24, the hollow core 26 witl in which the plunger operates, and the series winding 27 which partially embraces core 26. This winding 27 indicated as a single electrical conductor of relatively large cross-section and this conductor may form a part of the main motor circuit. The switch member 16 is biased to its closed position by the weight of the previously mentioned plunger and by means of the biasing spring 28 which surrounds a rod 29 secured to the lower end of the plunger. The lower end of this red 29 is pivotally connected with the lever 20, as indicated in both Figs. 1 and 2. The timing electromagnet is preferably as indicated in Fig.2, which is essentially the same as the corresponding electromagnet in Fig. 1. It will be understood from what has been said previously, that the armature 19 of this electromagnet is intended to move to its attracted position and serve as a stop for the lever. 20 so as to holdthe relay contact member 16 in its open position after th s contact has been operated thereto in respons e to an overload in the motor circuit. The armature prevents the contact member from moving from its full open position until the armatureis released. This is an advantage since the overload device can be calibrated exactly for a certain pick up and drop outvalue. If the plunger of the overload device werepern'iitted to move 'at once from its full operated position after the occurrence of an overload, the reluctance of the magnetic circuit would have changed and calibration difliculties would be encountered. As before explained, the timing action of the electromagnet 18 is secured by disconnecting the winding 21 from the source of supply and including this winding in a local circuit when the resistor 12 has been reinserted in the motor circuit. The various parts of the relay which control the resistor contactor 13 are preferably mounted on a slate or other insulating base 30, as shown in Fig. 2.

\Vhat I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination in a system of motor control, amotor circuit, a resistor therein, a switch member operable between an open position and a closed position for effecting the 'short-circuiting of said resistor and the reinsertion thereof in the motor circuit, means for effecting operation of said switch member, an electromagnet operably associated with said switch member so as to hold said switch member in one position and means operated by said switch member in moving to said position for controlling the energization of said electromagnet to release said switch member after a time interval.

2. In combination in a system of motor control, a motor circuit, a resistor therein, a contactor for shunting said resistor, means for controlling said contactor responsively to the motor current so that when the motor current is above av predetermined value said means moves to a position to effect a deenergization of said contactor whereby said resistor is reinserted in the motor circuit, an electromagnet operably associated with said control means so as to hold said control means in said position, and means operated by said contactor in moving to reinsert said resistor for controlling the energization of said electromagnet to release said control means after a time interval.

3. In combination in a system of motor control, a motor circuit, a resistor included therein, electroresponsive means for shunting said resistor, a switch having a switch member for controlling said means, the said switch member being normally in a position to effect the shunting of said resistor by said means while the motor current is less than a predetermined value and operative to another position to effect operation of said means to reinclude said resistor in the motor circuit when the motor current rises above said predetermined value, a normally energized electromagnet operably associated with said switch member so as to hold said switch member in said other position and means operated by said electro-responsive means in operating to reinclude said resistor in the motor circuit for controlling the energization of said electromagnet to release said switch member after a time interval.

4. In combination in a system of motor control, a motor circuit, a resistor therein, a contactor which when closed shunts said re sistor, a relay energized responsively to the motor current and having a switch member normally in the closed position which is operated to the open position thereof to effect the opening of said contactor when the motor current is above a predetermined value, a

normally energized electromagnet operably associated with said switch member so as to hold said switch member in its open position, said electromagnet being connected to be shunted by the opening of said contactor so as to delay the closing of said switch member during the interval in which the motor current is temporarily reduced by reason of the rcinsertion of said resistor.

5. In combination in a system of motor control, a motor-circuit, a resistor therein, a contactor for shunting said resistor, a relay energized responsively to the motor current and having a switch member normally in the closed position which is operated to the open position thereof to effect the opening of said contactor when the motor current is above a predetermined value, and an electromagnet having a normally energized winding and a magnetic movable member which moves to the attracted position so as to hold said switch member in its full open position, the said winding being connected so as to be shunted by the opening of said contactor to thereby provide a time interval for the release of said magnetic member and the return of said switch member to the closed position thereof.

In witness whereof, I have hereunto set my hand this 29th day of October 1926.

BENJAMIN IV. JONES. 

